In an internal combustion engine, the air-fuel ratio of combusted gasses may be controlled to address emissions. One source of air-fuel ratio errors may be an actual fuel injector flow rate being different than an expected fuel injector flow rate. One way to adjust a fuel injector flow rate is described by Thomas et al in U.S. Pat. No. 7,765,991. Therein, an output of an exhaust gas sensor is used in a feedback path to evaluate the error between an actual air-fuel ratio and a desired air-fuel ratio, and a fuel injector pulsewidth is adjusted based on this error, thereby adjusting the fuel injector flow rate.
However, the inventors herein have recognized potential issues for such systems. As one example, when using direct injection with fuels which are supercritical during at least some engine operating conditions, adjusting the fuel injection rate based on exhaust gas measurements may be unreliable. This is because in the supercritical state, the fuel's density within a sample can vary significantly and in a complex manner, which can result in unreliable exhaust gas measurements and thus inaccurate air-fuel ratio estimates. Supercritical fuels may effectively clean soot deposits from engine components, and direction injection of supercritical fuels may maintain engine cleanliness despite unpredictable injection properties.
In one approach, the issues above may be addressed by a method for a fuel system, comprising: adjusting a fuel injector control parameter based on an exhaust air-fuel ratio, selectively adjusting the fuel injector control parameter further based on a fuel pump gain, and operating a fuel injector based on the fuel injector control parameter. In this way, a desired amount of fuel injected may be accurately controlled over a wide range of engine temperatures and pump pressures, thereby ensuring desired air-fuel ratios. Furthermore, by allowing the fuel to be in a supercritical state at the fuel injector, advantages of supercritical fuels such as cleanliness of injector components and low soot emissions may be achieved while also achieving the desired air-fuel ratio.
As one example, a relationship between a volume of fuel delivered to the fuel rail by the direct injection fuel pump and a resultant increase in fuel rail pressure may be determined. This relationship may herein be referred to as a fuel pump gain, and may be based on an average of fuel pump gain estimates over a plurality of commanded pump strokes. In this way, an amount of pressure in the fuel rail may be associated with a fuel mass. A change in pressure upon an injection event may be measured and associated with an injected fuel mass based on the fuel pump gain. In response, an injector control parameter such as an injection gain may be increased, thereby increasing the commanded injection volume. By adjusting an injector gain based on the fuel pump gain pumping subcritical fuel, a desired injection mass may be delivered even when fuel density at the injection site is not predictable. For example, determinations of supercritical or subcritical fuel at the pump can be used to adjust operation and maintain improved air-fuel ratio control in the engine.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.